What version of ArcGIS? Which search has disappeared - the Find button to find records in layers in your map, or the Search window to find tools and datasets? The Search Window can be re-added by Windows menu > Search
– Midavalo♦Apr 22 '17 at 16:45

It sounds more like Find but it would be nice to be sure before spending time writing an answer.
– PolyGeo♦Apr 22 '17 at 20:32

I have now updated the question. I'm asking about the search tool and no it can not be re-added from the Windows menu.
– ArcCandleApr 30 '17 at 9:42

1

Looks like a bug. Have you contacted ESRI's support?
– BERANov 1 '17 at 12:39

1

Did you try the usual tech support approach of renaming your ESRI application data folder to see if that fixed it? It would be something like C:\Users\xxyourusernameherexxx\AppData\Roaming\ESRI? A less radical approach would be to just rename your Normal.mxt which would be in the subfolder \Desktop10.5\ArcMap\Templates. If that doesn't fix things you can rename them back to keep any customizations, and move on to other methods. Another similar tech support suggestion would be to rename your ESRI folder in the registry but messing with the registry may not be to your liking.
– johnsNov 1 '17 at 13:21

I have recently encountered similar problem in ArcGIS 10.5. I applied many possible solutions including renaming 'CATID' folder. However, it didn't worked for me. Alternatively, I logged off to new user account and found 'Search Tool' working properly.

You may copy your data from existing user account, create new windows user account and delete the existing one (optionally). The Search tool would become alive by clicking 'Search tool' button in ArcGIS.

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